feature: allow hard-coded characters in filternask masks

[roycewilliams]

filtermask doesn't appear to properly process hard-coded strings in masks:

$ pack2 filtermask ?l0 test.list
[snip]
a
c
d
e
f
g
h
l
m
n
p
q
s
t
u
v
x
y
z

$ pack2 filtermask ?l00 test.list
thread 'main' panicked at 'index out of bounds: the len is 1 but the index is 1', /rustc/8d69840ab92ea7f4d323420088dd8c9775f180cd/src/libcore/slice/mod.rs:2842:10
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace